As top-coating paint for automotive finish, there have heretofore been used those of solvent-based type in which a base resin (base polymer) component such as acrylic resin, polyester resin or alkyd resin having such functional groups as hydroxyl group and carboxyl group is used together with a curing agent such as amino resin, optionally-blocked polyisocyanate compound and epoxy compound.
For the protection of terrestrial environment, paint of low volatile organic matter content (VOC) has been demanded in these years. In the field of paint for automotive finish, colored base coating paint in particular has high discharge of VOC. It has therefore been strongly required to switch colored base coating paint to water-based type. It is now expected that, in future, solvent-based coating type will be replaced with water-based coating type.
Problems incidental to the switching of colored base coating paint to water-based type are, for example: (1) pigment dispersing resin blended in water-based paint is incompatible with the binder component in water-based paint; (2) pigment dispersing resin or dispersant fails to uniformly disperse the pigment, and invites flocculation or agglomeration of pigment; and (3) pigment dispersing resin or dispersant in the water-based paint composition fails to cure under baking, and invites the degradation of coating film performance.
Pigments which are used for colored base coating paint generally have hydrophobic surfaces, and are difficult to be wetted with water, while pigment dispersing resins which have conventionally been used for solvent-based colored paint have low solubility in water, and are inferior in dispersing stability in aqueous media. Hence, it is important in designing a pigment dispersing resin for water-based paint to secure satisfactory levels of both wettability of pigment and dispersing stability of pigment dispersing resin in an aqueous medium.
Besides, pigment is now being substituting for dye as a color material for ink, in particular ink-jet ink, from the viewpoint of water resistance and weatherability. However, water-based pigment ink is still unsatisfactory in the print density of printed image. Besides, pigment has a problem that, when its dispersion stability is low, clogging occurs in head when it is used for ink-jet ink. Hence, high-level pigment wettability and pigment dispersion stability are required of said water-based pigment ink.
Low viscosity and low molecular weight of pigment dispersing resin are considered advantageous for wetting property of pigment; and easy formation of steric repelling layer on the pigment surfaces or good solubility of the pigment dispersing resin in the continuous phase (aqueous medium) are considered advantageous for the dispersing stability. Both the pigment-wetting property and the dispersing stability of the pigment dispersing resin are considered to participate in wetting of the pigment surface with the pigment dispersing resin (a resin having a group capable of forming weak flocculation in aqueous system such as alkyl-group is effective) and in adsorption onto the pigment, which are attributable to the interaction between hydrophobic part on the pigment surface and that of pigment dispersing resin.
Past development of pigment dispersing resins or assistant for water-based paint or water-based ink has been advanced based on the foregoing viewpoints. For instance, Japanese Patent Application Laid-Open No. 154328/1975 proposed use of a straight chain anionic polymer containing acidic functional groups which is obtained through polymerization of a monomer mixture containing as a part of its monomer components an acidic functional group-containing, polymerizable unsaturated monomer such as (meth)acrylic acid, as the pigment dispersing resin. Said polymer, however, exhibits high solubility in aqueous media and hence is unsatisfactory in respect of dispersion stability of pigment dispersions. Furthermore, pigment dispersions formed with the use of said polymer show excessively high viscosity and are subject to a drawback of difficult handling.
Japanese Patent Application Laid-Open No. 182304/1989, Japanese Patent Application Laid-Open No. 316240/1995 and Japanese Patent Application Laid-Open No. 502097/1998 disclosed, as the pigment dispersing resin, a graft copolymer obtained through copolymerization of carboxyl-containing macromonomers. Said graft copolymer excels in pigment dispersing stability because its trunk portion is hydrophobic and the branch portion is hydrophilic whereas pigment dispersions formed with the use of said copolymer show high viscosity, and the graft copolymer is far from being satisfactory in view of the increasing demand for pigment dispersions having high pigment concentration to save cost and to reduce VOC.
Japanese Patent Application Laid-Open No. 255728/1997, Japanese Patent Application Laid-Open No. 267034/1997 and Japanese Patent Publication No. 19201/1996 disclose an idea of using, as pigment dispersing agent, a nonionic surfactant having polyoxyalkylene chain. Although capable of existing stably in water-based pigment dispersions, said surfactant is unsatisfactory in adsorption to pigment, and is in particular poor in let-down stability, and, moreover, inferior in the appearance of resultant coating film.
On the other hand, top-coating paint for automotive finish are now required to have not only film performance such as high durability, acid resistance, washability (scratch resistance) and chipping resistance, but also still better appearance of coating film in sharpness, transparency, color development, and the like, than before.
An object of the present invention is to provide pigment dispersing resins which excel in wetting property and dispersing stability even at high pigment concentration, and which can provide aqueous pigment dispersions having low viscosity, excellent in color developing property and giving a coating film with good appearance.
A further object of the present invention is to provide a water-based pigment dispersion which has low viscosity and which excels in color-developing property.
Another object of the invention is to provide a water-based paint which excels in curability and which is capable of forming a painted film of splendid finish appearance in sharpness, transparency and color-developing property and weatherability.
Another object of the invention is to provide a water-based ink composition which excels in sharpness, transparency, color-developing property and weatherability.
Other objects and characteristics of the invention will become apparent from the following descriptions.